1. Field of the Invention
This invention relates to a process for ammonia reduction-denitration of nitrogen oxides and an apparatus therefor, and more particularly it relates to a denitration process suitable for controlling the concentration of nitrous oxide (dinitrogen oxide, N.sub.2 O) by-produced at the time of denitration to a low level and also reducing nitrogen dioxide (NO.sub.2) with ammonia, and an apparatus therefor.
2. Description of the Prior Art
The process of subjecting nitrogen oxides (NO.sub.x) contained in exhaust gases to ammonia reduction with a catalyst into harmless nitrogen and water, i.e. the so-called catalytic ammonia reduction-denitration process, has a number of merits such as simple structure of apparatus, and has been practised for various purposes such as denitration of exhaust gases from combustion apparatus e.g. large scale boilers. Further, as the catalyst used for the above process, those composed mainly of oxides of titanium (Ti), vanadium (V), tungsten (W), molybdenum (Mo), etc. and having superior performance and life together have been practically used.
However, according to the studies of the present inventors, it has been clarified that the denitration process using the above catalysts is effective in the case where the nitrogen oxides consist of nitrogen monoxide (NO) or in the case where they consist of 50% or more of NO and 50% or less of nitrogen dioxide (NO.sub.2), but the process is not so effective in the case where they consist mainly of NO.sub.2 or in the case where they contain nitrous oxide (N.sub.2 O). This will be explained referring to the accompanying drawings.
FIG. 1 shows results obtained when an exhaust gas having a proportion of NO.sub.2 in NO.sub.x of 80% (gas composition: NO.sub.2 800 ppm, NO 200 ppm, NH.sub.3 1,500 ppm, O.sub.2 20%, H.sub.2 O 2%) was subjected to NH.sub.3 reduction in the presence of conventional catalysts. In this figure, numeral 1 represents the case of TiO.sub.2 -Co.sub.2 O.sub.3 catalyst and 2 represents the case of TiO.sub.2 -V.sub.2 O.sub.5. As apparent from this figure, it is true that NO.sub.2 and NO in the exhaust gas can be reduced, but on the other hand, N.sub.2 O is by-produced in a large amount, so that the substantial denitration performance is low. Thus, when the NH.sub.3 reduction-denitration process is applied to nitrogen oxides consisting mainly of NO.sub.2, removal of N.sub.2 O by-produced raises a serious problem.
Further, FIG. 2 shows results obtained when N.sub.2 O (gas composition: N.sub.2 O 1,000 ppm, NH.sub.3 1,000 ppm, O.sub.2 20%, H.sub.2 O 2%) was subjected to NH.sub.3 reduction in the presence of conventional denitration catalysts (numerals 1 and 2 in this figure represent the same catalysts as the above and 3 represents TiO.sub.2 -WO.sub.3 catalyst). As apparent from this figure, conventional denitration catalysts are not only inert to NH.sub.3 reduction of N.sub.2 O, but also contrarily have a tendency of by-producing N.sub.2 O. Thus, when N.sub.2 O is contained beside NO or NO.sub.2 as NO.sub.x in the exhaust gas, N.sub.2 O remains as it is; hence in this case, too, removal of N.sub.2 O raises a serious problem.
The above-mentioned properties of conventional type catalysts have never been known, and the reason is considered as follows: the analytical method of N.sub.2 O is so difficult that N.sub.2 O by-produced in the denitration reaction has not been studied and the evaluation of the denitration performance has been directed mainly to NO. Thus, no example of NH.sub.3 reduction of N.sub.2 O has so far been attempted, and naturally any catalyst which is highly active to the reaction has never been found.
Any of exhaust gases from nitric acid plant, metal-acid washing factory, nitrate-thermal decomposition furnace, etc. contain a high concentration of nitrogen oxides NO.sub.x consisting mainly of NO.sub.2, and various processes for removing NO.sub.x have been studied from the viewpoint of countermeasure to public pollution or prevention of its effect on succeeding equipments. Among these processes, an alkali-washing process has now most often been practiced, but this process offers problems that the percentage of NO.sub.x removed is low and waste water treatment is required. Thus, development of a simple dry treatment process has been desired such as a catalytic ammonia reduction-denitration process yielding good results in the boiler flue gas denitration process.
However, when the catalytic ammonia reduction-denitration process is applied to the above exhaust gas, there is raised a problem that N.sub.2 O is by-produced in a large amount; hence such an application cannot be a practical process. This is due to the fact that most of NO.sub.x in the above exhaust gas is in the form of NO.sub.2 and in this respect, it has different properties from those of the boiler flue gas consisting mainly of nitrogen monoxide (NO). Namely, according to the study results of the present inventors, the reaction of NO.sub.2 with NH.sub.3 proceeds by the medium of elementary reactions expressed by the following equations (1), (1') and (2) and is collectively expressed by the following equation (3): ##EQU1##
Since this reaction mechanism does not resort to any catalyst species, it is considered that even if any known denitration catalysts are used, by-production of N.sub.2 O according to the equation (2) cannot be avoided so long as the process of NH.sub.3 reduction-denitration of NO.sub.2 is employed.
Further, it has been said that when highly reducing metals are washed with acids or dissolved in nitric acid, N.sub.2 O is generated in a large amount. As described above, a large number of plants need removal of N.sub.2 O including potential N.sub.2 O-generation sources from which N.sub.2 O is by-produced at the time of ammonia reduction-denitration; hence development of a process for selectively removing N.sub.2 O has been earnestly desired.
Thus, the present inventors made various studies on a process for reducing N.sub.2 O, and as a result proposed a process of converting a part of NO.sub.2 into NO by a catalytic thermal decomposition in advance of its denitration reaction to thereby adjust the composition of NO.sub.x in the exhaust gas to (NO)/(total NO.sub.x).gtoreq.0.5, followed by subjecting the resulting gas to conventional ammonia reduction-denitration reaction. This process is intended to have NH.sub.2 consumed by NO as seen in the following equations to thereby inhibit the advance of the N.sub.2 O by-production reaction of the above equation (2): ##EQU2##
This denitration process in a two-stage manner makes it possible to inhibit the N.sub.2 O by-production down to a level as low as several ppm and also is superior in principle, but the structure of apparatus as well as its control are complicated; hence a problem of its practical use is raised. Thus, development of a denitration process capable of subjecting NO.sub.2 to ammonia reduction at one stage as in the case of NO has been eagerly desired.
An object of the present invention is to provide a highly active catalyst for the reduction reaction of N.sub.2 O by means of NH.sub.3 and a process for ammonia reduction-denitration of N.sub.2 O by the use of the catalyst.
Another object of the present invention is to provide a denitration process capable of subjecting NO.sub.2 to NH.sub.3 reduction at one stage without by-producing N.sub.2 O, and an apparatus therefor.
The present inventors have made studies on the activities of catalysts consisting of various metal oxides, upon the reaction of N.sub.2 O-NH.sub.3 (see the following equation (6)): EQU 3N.sub.2 O+2NH.sub.3 .fwdarw.4N.sub.2 +3H.sub.2 O (6)
As a result, it has been found that any of known catalysts comprising as their active component, oxides of transition metals such as vanadium (V), tungsten (W), chromium (Cr), manganese (Mn), cobalt (Co), etc. cannot promote the ammonia reduction reaction of N.sub.2 O, whereas compounds obtained by replacing a part or the total of alkali metals or alkaline earth metals contained in zeolites such as mordenite, Y type zeolite, etc. by iron (Fe) or hydrogen have a high activity of the ammonia reduction of N.sub.2 O.